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386                              CHEMICAL ENGINEERING
in. The boiling point of the liquor goes up very rapidly during the final concentration. The liquors become acid during evaporation and therefore all parts of the evaporators coming in contact with the liquor must be made of acid-proof bronze with extra-heavy copper tubes. Vapor belts can be made of cast iron.
Bibliography and History.—In spite of the fact that the art of Drying is a very important branch of the chemical industry, there was up to a few years ago not a single textbook giving specific information and data on this subject. Only comparatively recently three books have appeared: E. Hausbrand, "Drying by Means of Air and Steam," Thos. C. Marlow, " Drying Machinery and Practice" (with a chapter, 'Bibliography of Drying and Dessicating'); and Otto Marr, "Das Trocknen and Die Trockner" (Drying and Drying Machinery). Marr's book is by far the most complete treatise on this subject, and it is to be regretted that it has not been translated into the English language, as it contains valuable data regarding the theory and practice of drying.
The art of Drying must naturally be a very old one, but the author has been unable to find any accurate information as to the invention and first application of the various types of dryers. Vacuum dryers were introduced only about 35 years ago by Passburg.
Theory of Drying.—Drying is the process of lowering the weight of a substance by reducing the amount of water or solvent contained in this substance. This may be accomplished by: Pressure, suction or decantation; evaporation, by means of hot air or gases, steam, hot water, oil and electricity. The application of pressure, suction or decantation can only remove a certain amount of •moisture, and in order to make an article bone dry, it is always necessary to resort to heat and evaporate the water or solvent. Filter presses, centrifugals, hydraulic presses, stationary and revolving suction filters have been treated in other parts of this book, and this chapter will only deal with drying by evaporation.
In order to reduce operating expenses, the excess of water should always be removed by pressure, suction or decantation as far as possible, because drying by evaporation is rather expensive, and requires costly machinery. For instance, table salt and granulated sugar are first handled in centrifugals and then completely dried in rotary dryers with hot air.
Drying by evaporation requires heat for three purposes: first, to raise the temperature of the product to the required level; second, to evaporate the water or solvent; third, to replace the losses by radiation which in most drying processes form a large percentage of the total heat required. In some cases, it is also necessary to include the heat required for the heating of the shelves, racks and wagons carrying the material. The heat may be applied directly by air and gases, or indirectly by steam, hot water, oil and electricity.
Drying by Air and Gases.—Capacity and efficiency of an air-heated dryer always depends on atmospheric conditions, and such apparatus should therefore be figured for extreme conditions: in winter with cold, dry air, and in summer with warm, almost saturated air. Weather conditions interfere often very